By default, Excel automatically calculates every formula automatically. This means that when values in a cell change, the formula recalculates immediately to display the correct information. However, there may be occasions when you do not want this to happen. The Calculation group allows the user to have flexibility in terms of when values are calculated. In most cases, the user should not change the Calculation Options from automatic, but it is nice that Excel provides the user the flexibility.
The first icon, Calculation Options contains a drop-down menu that allows the user to quickly change the calculation setting in the active workbook between Manual, Automatic and Automatic except for Data Tables.
The Calculate Now icon (F9) allows the user to recalculate all formulas in a workbook if the calculation setting was set to manual.
Likewise, the Calculate Sheet icon- (Shift + F9). Allows the user to update all of the calculations in a worksheet if calculation settings were sent to manual.